This invention relates generally to desalters and more particularly concerns a vessel and method for removing water, salt and gas from oil.
The fluid produced from a typical oil well commonly includes gases and is also often tainted by salt, especially if the well has relatively low downhole pressure and is therefore susceptible to migration of salt water into the oil reservoir. The salt is typically removed from the oil by mixing fresh water with the fluid and then remove the resulting saline solution. The efficiency in removing the salt water is sometimes improved by the addition of chemicals or heat to the emulsion. An elaborate array of equipment and a generally inefficient method have evolved in the industry. As is illustrated in FIG. 1, the oil is first admitted into a separator to remove gas. It is then heated in an indirect heater to approximately 175.degree. F. Gases are again removed from the heated oil in another separator. In many applications, globules of water which have not associated with the oil are then removed in a free water knock out. Fresh water is then introduced into and mixed with the oil, the result being a combination of residual gases, globules of oil, an emulsion of oil globules in salt water film casings and globules of free water, all substantially separable into tiers by gravity. Some liquid will be dispersed in the gas, some gas will be entrained in the oil and salt may be dispersed throughout. This combination is then purified in a first desalter and, usually, in a second desalter. In some applications, it may be necessary to use more than two desalters. The purified oil is then delivered to a storage tank. This method and the known desalters used to accomplish it have many deficiencies.
A first deficiency is that, in known desalters, the fluid flows through a horizontal cylindrical vessel having an inlet at one end and an outlet at the opposite end, so that the fluid quickly flows in a single pass through the vessel. The benefits of longer residence times are disregarded.
A second deficiency is that known desalters are liquid-fluid packed and cannot be used for separation of gas.
A third deficiency is that, in the normal flow pattern of fluid through a vessel, high velocity flow occurs only in approximately the middle forty percent of the vessel cross sectional area as can be seen in FIG. 2. Outside the high velocity flow path, approximately twenty percent of the flow vessel cross-sectional area exhibits eddy current flow. The approximately forty percent of the cross sectional area remaining at the perimeters of the vessel outside of the eddy flow zones exhibits stagnant flow. Thus, very little of the vessel is put to efficient use.
A fourth deficiency is that for known desalters it is necessary to preheat the fluid to at least 175.degree. F. This drives off all the light ends entrained in the oil, especially gasoline, and shrinks the oil volume.
A fifth deficiency is that while 175 B.T.U.'s are required to raise the temperature of one barrel of oil 1.degree. F., it takes 350 B.T.U.'s to raise the temperature of one barrel of water 1.degree. F. But known desalters heat the fluid injected into them without first removing any of the free water injected into or separated by the desalter. The high temperature requirement, together with the need to raise and maintain not only the emulsion and purified oil but also the water to and at that high temperature, is a highly inefficient use of energy.
A sixth deficiency is that known desalters use vertically aligned high voltage grids in a very inefficient fashion to assist in breaking down the emulsion. In order to recover globules of purified oil from the emulsion, it is necessary to rupture or break the surface tension of the water films encapsulating the oil globules. For thinner films of water, known as tight emulsions, surface rupture is much more difficult. Therefore, known desalters immerse vertical grounded and high voltage grids in the fluid along the length of the vessel. High voltage cycled across the grids stretches the water film to a maximum at the peaks of the power sine wave. However, the vertically aligned grids are typically eighteen to twenty-four inches apart because the high quantity of water retained in the desalter would short the system if the grids were closer together. Furthermore, known desalter grid systems afford no adjustment for the different percentages of water content encountered in different emulsions. If the voltage applied to a given emulsion is too low, the water film may not be stretched sufficiently to break its surface tension. On the other hand, if the voltage applied is too high, the emulsion globules may be split into smaller emulsion globules rather than separated into oil globules and water globules. But each known desalter applies its preset spacing and voltage to all of its applications.
A seventh deficiency is that, since the grids are vertical, considerable portions of the flow path are vertical. This results in the separated water flow countering the flow of the crude oil, increasing the water settling time.
It is, therefore, an object of this invention to provide a desalter and a method of desalting oil which increases the residence time of the emulsion in the desalter. Another object of this invention is to provide a desalter and a method of desalting oil which more efficiently uses the flow area of the vessel. A further object of this invention is to provide a desalter and a method of desalting oil which uses heat more efficiently in breaking the emulsion. Yet another object of this invention is to provide a desalter and a method of desalting oil which reduces the process temperature requirements of the desalter. It is also an object of this invention to provide a desalter and a method of desalting oil which removes a substantial quantity of free water from the desalter before the application of heat to the fluid. Still another object of this invention is to provide a desalter and a method of desalting oil which increases the efficiency of the high voltage grid system of the desalter. An additional object of this invention is to provide a desalter and a method of desalting oil which eliminates much of the equipment presently used in conjunction with the desalter.